The present invention is directed to a bandpass filter formed on a dielectric substrate which reduces the transmission of unwanted noise.
Bandpass filters are often utilized in various types of communication equipment, for example, television receivers and cellular phones. Such filters discriminate between signals by passing signals in a desired, predetermined frequency band (i.e. pass the signal from the input to the output of the filter unattenuated), while preventing the transmission of signals outside the predetermined frequency band. In the event signals outside the desired frequency band are transmitted by the filter (i.e. the signals pass through the filter unattenuated), system performance is degraded.
FIG. 16 is a schematic plan view illustrating a prior art bandpass filter. Referring to FIG. 16, the prior art filter contains a substrate 1 formed from a dielectric substance. The dielectric substrate 1 functions as a support for three coplanar waveguide resonators 2-4, hereinafter referred to as a resonator, an input electrode 5, and an output electrode 6. Each resonator 2-4 comprises a strip conductor 2a-4a. The strip conductor 2a of resonator 2 is coupled to the input electrode 5 through capacitor 7. The strip conductor 4a of resonator 4 is coupled to the output electrode 6 through capacitor 8. Finally, strip conductor 3a of resonator 3 is coupled to the strip conductor 2a of resonator 2 and strip conductor 4a of resonator 4 through capacitors 9 and 10, respectively.
As is well known in the art, the frequency response of a filter defines the level of attenuation of an input signal over the entire frequency spectrum. The frequency response of the prior art bandpass filter of FIG. 16 is illustrated in FIG. 15. Specifically, FIG. 15 depicts the level of attenuation of a signal by the prior art filter which has a predetermined center frequency equal to fo. The line designated by the letter "A" represents the minimum level of attenuation acceptable for signals having a frequency outside a predefined frequency bandwidth of the filter. Frequency values 3 fo, 5 fo and 7 fo represent harmonic components of the center frequency, fo, of the filter.
As is apparent from FIG. 15, signals having a frequency equal to the harmonic components 3 fo, 5 fo and 7 fo are not adequately attenuated by the prior art filter. Although not shown, higher odd multiple harmonic components of the center frequency of the filter (i.e. 9 fo, 11 fo, etc.) also traverse the filter without adequate attenuation. Thus, the prior art filter does not sufficiently attenuate all signals having a frequency outside the filter bandwidth centered about fo, which results in the generation of noise and the degradation of the performance of a system utilizing such filters.
For example, typically a transmitter broadcasts signals of a single predetermined frequency, fo. This selective transmission is accomplished by filters, which attenuate signals having a frequency other than fo. However, if the prior art filter is used in such a transmitter, the transmitter will also output signals having a frequency equal to the odd harmonic components of fo, thereby transmitting unwanted signals (i.e. noise). Similarly, if the prior art filter is used in a receiver, it will allow for the receipt of unwanted signals having a frequency equal to the harmonic components of the center frequency of the filter (i.e. noise).
Accordingly, there exists a present need for a bandpass filter comprising coplanar waveguide resonators formed on a dielectric substrate which exhibits a frequency response that adequately attenuates the harmonic components of the center frequency of the filter so as to prevent the generation of noise.